Different types of self-locking nuts are known.
For instance U.S. Pat. No. 4,893,977 discloses a self-locking nut, wherein “V” or “U” shaped slots are provided in the cylindrical end, said slots being arranged for being elastically compressed by a radially operating external helical spring.
U.S. Pat. No. 5,160,227 discloses a self-locking nut of the type cited above, wherein, in order to improve the corrosion strength performances and to allow use at temperatures over 250° C., it is provided that the helical spring is made of stainless spring steel, such as the so-called AISI (American Iron and Steel Institute) 302 steel.
The Applicant has noted, in general, that, as a result of the technological evolution of different devices in terms of increased speed of moving parts, increased vibrations of such parts and increased operating temperatures, there is a growing demand for nuts having, in particular, high braking torque when disassembled (unscrewing braking torque) for use both at ambient temperature and at high temperatures.
The known nuts in which the spring is made of stainless spring steel such as the AISI 302 steel, though having a very high braking torque at their first unscrewing, show, as illustrated in FIGS. 1 and 2 (corresponding to use at ambient temperature, e.g. from −50° C. to 100° C.) and in FIGS. 3 and 4 (corresponding to use at high temperature, e.g. from 300° C. to 400° C.), a progressive decay of the features or measures of braking torque in Newton per meter (Nm) when the number of screwing/unscrewing cycles increases, with the consequence that their re-use is substantially not very reliable.
In FIGS. 1 and 2 assembling and disassembling braking torque (screwing torque) values as measured in tests at ambient temperature in 35 screwing/unscrewing cycles of two samples of nuts M12 and M16, respectively, are shown. In FIGS. 3 and 4 assembling and disassembling braking torque (screwing torque) values as measured in tests at high temperature in 35 screwing/unscrewing cycles of two samples of nuts M12 and M16, respectively, are shown.
As it appears clear from the values, the known nuts do not allow to meet the requirements to maintain the performances of the nuts constant and high in terms of braking torque, in particular in those technological contexts in which the nuts must be reused safely on parts, so-called wear parts, that require a high number of replacements in the life period of the devices of which they are part.
An example of such use can be a wear part for a vehicle in which it is provided that, after every replacement of the consumed part, the new mounted part maintains an high operating reliability.
This problem, of course, exists both for use at ambient temperature (FIGS. 1 and 2) and for use at high temperature (FIGS. 3 and 4).
The Applicant has substantially noted that as a result of the technological evolution, in current practice the known nuts, when applied, in particular, for fastening wear parts, involve the problem that the replacement of the consumed parts is also associated necessarily to the replacement of the nuts in order to avoid the risk of performance decay of the nuts themselves.
Therefore there exists the technical problem of avoiding the replacement of the nuts following, for instance, maintenance operations on wear parts because the replacement of the nuts involves both high maintenance costs and material waste; this last is a problem as to which the sensibility of the market has grown in an optical of limitation in the exploitation of the energetic resources.